1. Field of the Invention
The present invention relates to novel active alumina agglomerates and to a process for the production of such novel agglomerates.
This invention more especially relates to novel porous active alumina agglomerates having a bimodal type of pore distribution.
2. Description of the Prior Art
It is known to this art that porous alumina agglomerates have highly desirable mechanical properties. These agglomerates find applications in various adsorption processes, but are used primarily in the field of catalysis. Indeed, heterogeneous catalysis often requires the use of catalyst supports having a large active specific surface area, on the order of several hundreds of square meters per gram, and having a high porosity, provided by pores with diameters on the order of several tens to several hundreds of Angstroms.
Furthermore, in order to facilitate fluid exchange and to improve the diffusion of fluids into the active volume of the catalyst support, processes for the preparation of alumina agglomerates having two types of pores have been proposed to this art. Such agglomerates have, on the one hand, pores with dimensions less than 1,000 .ANG. and constituting a microporous component, and, on the other, pores with dimensions of greater than 1,000 .ANG. which constitute a macroporous component.
Macroporosity permits a more rapid and easier exchange between the external medium and the compounds comprising the catalyst, while the microporosity assures the activity of the catalyst.
FR 1,386,364 describes the preparation of alumina agglomerates having great mechanical strength. According to this '364 patent, the process entails treating the active alumina agglomerates in an autoclave in the presence of water, and then drying and calcining the resulting products at a predetermined temperature in order to provide the desired specific surface area and particular pore dimensions.
It is also possible to improve the mechanical strength of the agglomerates by treating them in an autoclave in the presence of an acid, as described in U.S. Pat. Nos. 3,628,914 and 3,480,389.
FR 1,383,076 describes a process for preparing alumina agglomerates having a controlled porosity with a high crush strength and resistance to attrition. This process includes impregnating the porous active alumina in granular form, produced by dehydrating alumina hydrates in a flowstream of hot gases, with such amount of water as is necessary to fill the pores of the alumina, and then shaping, aging and reactivation of the agglomerates. Prior to shaping, a material such as naphthalene may be added, which disappears completely during heating, in particular during the heating for purposes of reactivation, to create the pores having large dimensions (macroporosity).
To reinforce the strength of the material, FR 2,496,631 proposes treating the agglomerates by subjecting them to a hydrothermal treatment in a confined space, in the presence of an acid.
In this manner, the resulting agglomerates are impregnated with an acid solution and are maintained at a certain temperature in a confined space.
This hydrothermal treatment in the presence of an acid makes it possible to reinforce the mechanical properties of the agglomerates, but presents the major disadvantage of disrupting the pore distribution of the treated agglomerates, in particular by causing the at least partial disappearance of the macroporosity.